1. Field of the Invention
This invention relates to an improved analog to digital (A/D) conversion circuit. More particularly, it relates to such an A/D conversion circuit which operates in a parallel mode, commonly known as a flash converter. Most especially, the invention relates to such an A/D converter in which the number of bits of resolution achievable by the A/D converter when implemented in integrated circuit form is not limited by the complexity limit of the circuit which can be fabricated in integrated circuit form.
2. Description of the Prior Art
High speed integrated circuit A/D converters have been provided in the prior art for the purpose of rapid conversion of analog signals to digital form for digital processing. The state of the art in such flash A/D converters is summarized by Beresford, R., Ed., "High Speed Data Conversion", Electronics, May 4, 1982, p 134-142. The state of the art in such flash A/D converters is further indicated in the following papers from the Proceedings, 1982 Wescon; Kauffman, S., "Multistage Error Correcting A/D Converters", Paper Number 30/1; Olsen, R., "Architecture and Applications of A/D Converters", Paper Number 30/2; and Wilson, G., "The Promise of Bipolar VLSI for High Speed A to D Converters", Paper Number 30/4.
As set forth in this prior art, conventional A/D converters with n-bit resolution require (2.sup.n - 1) comparators, or 255 comparators for an 8 bit resolution flash converter, and 1,023 comparators in the case of a 10 bit resolution flash converter. The necessity to provide such large numbers of comparators in these prior art flash converters means that 8 bit resolution is the limit in currently available integrated circuit flash A/D converters, with 10 bit resolution converters being considered for future products with further improvements in integrated circuit fabrication technology. Further improvements in flash A/D converter design are necessary in order to remove integrated circuit fabrication technology as a limiting factor in the resolution of integrated circuit flash A/D converters.